1. Field of the Invention
The invention relates to a frequency diversity system for mobile reception of high frequency UHF or VHF signals. In particular, the invention relates to a frequency diversity system for mobile television reception in the meter or decimeter wavelength.
2. The Prior Art
Frequency diversity, especially as it relates to television reception, has not as yet been fully established. Thus, there are no receivers which automatically select between two emitted frequencies corresponding to one television program within a certain coverage area, and which are able to switch the reception to a channel with superior signal quality. Nor is there the possibility of a supra-regional selection between more than two different frequencies with one television program, such as has been used for radio in the form of Radio Data Systems (RDS). For radio, RDS, besides spacial diversity, is a basis for interference-free and continuous high-quality reception during long car trips. To date, nothing comparable is available for television broadcasting.
In television technology, the normal system uses a predetermined number of transmitters with one frequency, each stored inside the receiver, whereby the frequency can be accessed manually by selecting a certain receiving channel. This principle will not work for television reception in moving vehicles, at least at the boundaries of the coverage area, due to the weakening of the actually-received signal, and also for other well-known reasons, such as fading, multipath reception, interferences, etc.
Moreover, it is known that diversity circuits may cause a deterioration of the trueness of the picture or reproduction, i.e., interference noises during changeover between antennas or frequencies, especially during switching back and forth between signal paths, when the signal quality on the available channels is approximately the same. This is especially true for the more complicated operational conditions of television compared to audio radio. Thus, in addition to continually providing a signal with sufficiently high signal level and signal-to-noise ratio, a stable field synchronization as well as a synchronization between image and sound must be guaranteed. Thus, whenever the diversity principle is to be utilized in a moving vehicle to improve reception, care must be taken to ensure that the advantages obtained through greater expenditure will not be impeded by defects in the diversity function itself.